1. Field of the Invention
This invention relates to lens systems, and more particularly, to lens systems having a focusing provision in the lens group at the rear part thereof, or employing the so-called rear focusing method.
2. Description of the Prior Art
In the past, for preforming focusing, there have been made proposals such as those for moving only one lens group of the rear part of a lens system, or the so-called rear focusing method, or for moving a central lens group of a lens system, or the so-called inner focusing method.
These focusing methods are suited for making the lens system more compact, and also provide the advantage that the focusing operation can be easily performed.
On the other hand, the use of only the rear part of the lens system in focusing generally results in deteriorating the image quality. To avoid this, therefore, it has been the prior art practice that the focusing lens group is divided into two parts which are moved at different speeds from each other to effect focusing, or the so-called floating method is employed, or that the diaphragm mechanism is made to move together with the lens group when focusing. However, such focusing methods generally have a disadvantage that the structure of the lens mounting becomes complicated. Particularly in the application of the rear focusing method to lens systems having focal lengths of from 100 mm to 200 mm for 35 mm format, or so-called semi-telephoto lenses, it was observed in many cases that the tendency of image quality loss was increased.
Reference is made to lens systems having a plurality of lens groups and having the focusing provision besides the front or 1st lens group, or employing the rear focusing method and the inner focusing method, such as those disclosed in U.S. Pat. Nos. 3,408,135, 3,854,797, 4,310,211 and 4,348,084 and Japanese Laid-Open Patent No. Sho 56-27108.